A fullstack but simple mail server (smtp, imap, antispam, antivirus...). Only configuration files, no SQL database. Keep it simple and versioned. Easy to deploy and upgrade.
If you had a domain name and only wanted to have one(or a few) email address from this domain, but you want to forward all the emails to another email account. SMF is exactly what you need.
A little docker image containing everything you need to test or run Zarafa Web Meetings.